The Yuganaddha Sutta lays out the four sort of permutations of the development of Samatha and Vipassana as:

"There is the case where a monk has developed insight preceded by tranquillity. As he develops insight preceded by tranquillity, the path is born. He follows that path, develops it, pursues it. As he follows the path, developing it & pursuing it — his fetters are abandoned, his obsessions destroyed.

"Then there is the case where a monk has developed tranquillity preceded by insight. As he develops tranquillity preceded by insight, the path is born. He follows that path, develops it, pursues it. As he follows the path, developing it & pursuing it — his fetters are abandoned, his obsessions destroyed.

"Then there is the case where a monk has developed tranquillity in tandem with insight. As he develops tranquillity in tandem with insight, the path is born. He follows that path, develops it, pursues it. As he follows the path, developing it & pursuing it — his fetters are abandoned, his obsessions destroyed.

"Then there is the case where a monk's mind has its restlessness concerning the Dhamma [Comm: the corruptions of insight] well under control. There comes a time when his mind grows steady inwardly, settles down, and becomes unified & concentrated. In him the path is born. He follows that path, develops it, pursues it. As he follows the path, developing it & pursuing it — his fetters are abandoned, his obsessions destroyed.

The first and the last one seem fairly clear. In the first category, one practices Samatha Bhavana to the point of attaining either to access concentration or any of the aramanupanijhanas, and then practices Vipassana Bhavana starting with the jhana factors as described in the begining of Chapter XVIII of the Visuddhimagga. In the last category, the meditator moves beyond the Vipassana Upakilesa.

However, I don't understand the difference between the second and third. What does it mean specifically to develop Samatha and Vipassana together as opposed to developing Vipassana first? What does the commentary have to say in defining these two terms?

The non-doing of any evil,
The performance of what's skillful,
The cleansing of one's own mind:
This is the Buddhas' teaching.

Vipassanāpubbaṅgamaṃ samathaṃ. Mp: “This refers to one who by natural bent first attains to insight and then, based on insight, produces concentration.” In the commentarial literature this is called one who makes insight the vehicle (vipassanāyānika). See Vism 588; Ppn 18:4.

Samathavipassanaṃ yuganaddhaṃ. In this mode of practice, one enters the first jhāna and then, after emerging from it, applies insight to that experience, i.e., one sees the five aggregates of the jhāna (form, feeling, perception, etc.) as impermanent, bound up with suffering, and nonself. Then one enters the second jhāna and contemplates it with insight. One applies the same procedure to the other jhānas as well until the path of stream-entry, etc., is realized

When you read S.N. or A.N. You should connect sutta that come before or after together, too. Obviously, the sequence of everything in contexts are very important to understand tipitaka. This is a reason that why I often said that reciting&memorizing, just pāli, are better than reading, and especially better than reading the translated version.

I don't want to insult anyone. I just try to advise the one way path that intention-cetasika of bodhisatta choose to born to be then buddha force bikkhu to make it done for to be a perfect bhikkhu and for to be ariya (perfect person).

The Answer

Full answers already answered inside yugganaddhakathā of paṭisambihdāmagga, and nettipakaraṇa desanāhāravibhaṅga+nayasamuṭṭhāna. Also, you can use commentary, but you must recite and memorize tipitaka first because comment in commentary is over short, for the reader.

Practitioner who hard to meditate, quickly/delayed access to insight=Development of insight preceded by meditating concentration. This practitioner's hindrances often arise between his meditation. He must inhibits hindrances before meditate an insight. So buddha said "Bhikkhu, you must meditate a concentration, bhikkhu who meditated a concentration, will access to insight (5 aggregates)".

Practitioner who easily to meditate, quickly/delayed access to insight=Development of concentration preceded by meditating insight. This practitioner's hindrances not often arise between his meditation. So he meditate easily.

Practitioner who quickly access to insight, easily/hard to meditate=Tranquility developed in tandem with insight. This practitioner turned to be pro of 1st and 2nd above. He meditating insight at pahāna-pariññā state (from 3 pariññā).

Practitioner who delayed access to insight, easily/hard to meditate=Practitioner who has to inhibit hindrances from mind when meditating insight. This practitioner is not pro enough of 1st and 2nd above. He still meditating insight at tīraṇa-pariññā state. So he have to try more to meditate 1st, 2nd to be 3rd. See the advance detail in tipitaka paṭisambhidāmagga (somewhere, I have not read it done), and visuddhimagga understanding-part maggāmaggañāṇadassanavisuddhi-niddesa to (especially from indriyatikkhakāraṇanavaka-kathā to paṭipadāñāṇadassanavisuddhi-niddesa).

Vocabulary in sutta that connect it together:

Paṭipadā=Maggo sañjāyati.

Dhammuddhacca=Vipassanpakilesa.

(there are more word, but you should to recite&memorize&meditate from pali of them yourselves with the meditated teacher such as pa-auk teachers).

Special extended Q&A:

Q:Are you saying that meditation comes easily for one who develops insight before concentration, because insight overcomes hindrances?

A:No, that is thai understanding tradition which not compatibility with the suttas' explanation in that vagga. Buddha explained about 5 indriya, so it is about power accumulation of whole 5 indriya. It is not about one help one such as paññindtiya (insight) help samādindriya (concentration), because no way that the practitioner can meditate an insight, before his hindrances overcome by concentration. So I quoted above that buddha said "Bhikkhu, you must meditate a concentration, bhikkhu who meditated a concentration, will access to insight (5 aggregates)."

Moreover, I can completely say that thai meaning is a mistake that make reader confuse. A person who was the beginner of that definition was not tipitaka memorizer (he was anti-abhidhammist, who had a manpower in thai about more than 100 years ago in colony war period). This is not my idea even though not commentary idea, because the sequences of pali words and sutta force the memorizer, who reciting pali, to understand like that.

Q:OK. So, it is the opposite. It is easier in meditation practice, to cultivate concentration before insight or both in tandem, but it's harder in meditation practice, to cultivate insight before concentration? And what about the fourth path that I asked in my question? Where does that come in?

Above message maybe out of date. Latest update will be in massage's link.--------------------------------------------------
Tipitaka memorization is a rule of monks. It isn't just a choice. They must done it.
bahussuto nāma tividho hoti – nissayamuccanako, parisupaṭṭhāpako, bhikkhunovādakoti. http://UnmixedTheravada.blogspot.com/20 ... monks.html

"Practitioner who has to inhibit (samatha) hindrances (dhammuddhacca) from mind when meditating insight (vipassanā, especially taruṇa-udayabyañāṇa). "
That is so, otherwise the insights will not be aligned with the path, but diverted, delusory and not productive of progress.

Getting back to basics, sila, samadhi and panna are formulated in that order because dynamically speaking, sila is the basis of samadhi and samadhi the basis of panna.